2D ultrasound give you outlines and flat looking images, but can be used to see the internal organs of the baby. This is helpful in diagnosing heart defects, issues with the kidneys and other internal issues. The 3D images are used to show you three dimensional external images which may be helpful in diagnosing issues such as a cleft lip. What 4D ultrasound brings to the table is that as the image is continuously updated, it becomes a moving image, like a movie.
Here are some examples:
About GE 4D Ultrasound. http://www.gehealthcare.com/usen/ultrasound/education/products/cme_3d4d.html. Last accessed 1/31/11.
Ultrasound: Then & Now. http://www.gehealthcare.com/usen/patient/ultrasound/ultra_thennow.html. Last accessed 1/31/11.